A guest on @BBCRadio4 has just said that any question about the viability of population growth is 'inherently racist'.

She then when on to use the 'green revolution' - which has devastated tropical rainforests and soil quality across the world - as an example of humans are good at solving problems.

When Marr asked the question of what should be done instead, there was of course the usual flannel. The average annual CO2 footprint of people in the U.S - let's not even talking about land and resource use - is 16 tonnes. What's the guests idea for how we get that to 1 tonne?